ultralytics/assets update master to main

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

GitHub PR modifies image source paths in the README and Jupyter Notebook files.

📊 Key Changes

• Updated image URLs from a master branch to main branch in various markdown (.md) and Jupyter Notebook (.ipynb) files.
• Adjusted the width for some social media logo images to maintain consistency.

🎯 Purpose & Impact

• The primary goal is to reflect the change of the default branch naming from master to main for the assets repository, ensuring that image links remain functional.
• This improves the repository maintenance and follows a recent trend in Git repository naming conventions for inclusivity.
• Impacts are mostly internal and won't significantly affect users, aside from ensuring images in the documentation continue to display correctly. 🖼️